1. Phytoplasmas

Definition

Phytoplasmas are wall-less prokaryotic pathogens restricted to the phloem tissue of plants and transmitted by phloem-feeding insects like leafhoppers, psyllids, and planthoppers. They are associated with several plant diseases, particularly those involving stunted growth, yellowing, and witches’ broom symptoms.

Characteristics

• Morphology: Pleomorphic (variable shape) due to the absence of a rigid cell wall.
• Size: Small (0.2–0.8 µm in diameter).
• Genome: Highly reduced, around 530–1350 kb, making them dependent on their hosts.
• Habitat: Found in the phloem tissue of infected plants and the bodies of insect vectors.
• Transmission:
  • Spread by phloem-feeding insects like leafhoppers and psyllids.
  • Can also be transmitted through grafting, infected cuttings, or vegetative propagation.

• Symptoms:
  • Yellowing (chlorosis)
  • Witches’ broom (proliferation of shoots)
  • Stunting
  • Floral abnormalities (e.g., phyllody—conversion of flowers into leaf-like structures)

Examples of Diseases Caused by Phytoplasmas

1. Grassy Shoot Disease of Sugarcane: Caused by Candidatus Phytoplasma sacchari.
2. Little Leaf of Brinjal: Caused by Candidatus Phytoplasma aurantifolia.
3. Coconut Lethal Yellowing: Caused by Candidatus Phytoplasma palmae.
4. Aster Yellows: Caused by Candidatus Phytoplasma asteris.
5. Sesame Phyllody: Caused by Candidatus Phytoplasma sesami.

2. Spiroplasmas

Definition

Spiroplasmas are wall-less, helical-shaped prokaryotes found in phloem tissue of plants and the hemolymph (blood) of their insect vectors. They are distinct due to their spiral or helical morphology and motility.

Characteristics

• Morphology: Helical or spiral-shaped and motile.
• Size: Small (0.2–0.3 µm wide and up to 5 µm long).
• Genome: Larger than phytoplasmas, approximately 1 Mb.
• Habitat: Found in the phloem of plants and hemolymph of insect vectors.
• Transmission:
  • Transmitted by leafhoppers and planthoppers.
  • Can be experimentally transmitted through mechanical inoculation.

• Symptoms:
  • Stunting
  • Yellowing (chlorosis)
  • Reduced vigor and sometimes wilting

Examples of Diseases Caused by Spiroplasmas

1. Citrus Stubborn Disease: Caused by Spiroplasma citri.
2. Corn Stunt Disease: Caused by Spiroplasma kunkelii.
3. Periwinkle Disease: Caused by Spiroplasma phoeniceum.

Comparison Between Phytoplasmas and Spiroplasmas